Abstract:

A germanate glass composition suitable for use in a fiber amplifier for
broadband amplification of optical signals is provided. The glass
preferably includes 35-75% GeO2, 0-45% PbO, 5-20% BaO, 5-20% ZnO,
and 2-10% R2O (R=Na, Li, K). It is doped with thulium ions
(Tm3+) and codoped with holmium ions (Ho3+). The glass
composition of the invention results in a remarkably large bandwidth as
compared with previous glasses. It is also highly compatible with
existing silica optical fibers.

Claims:

1. A germanate glass composition for optical fiber amplification being
doped with Tm3+ and Ho3+ and further comprising at least 35
mole % GeO2 and oxides of Ba, Zn and R, R being selected from the
group of Na, Li, and K, for broadening the amplification wavelength band
associated with the glass composition.

2. The glass composition according to claim 1, further comprising up to 45
mole % PbO.

3. The glass composition according to claim 1, wherein the glass component
amounts are such that the amplification wavelength band is at least 250
nm.

8. An optical amplifier fiber (10) comprising a core (12) surrounded by at
least one cladding (14), wherein the core is at least partly formed of a
germanate glass composition doped with Tm3+ and Ho3+ and
further comprising at least 35 mole % GeO2 and oxides of Ba, Zn and
R, R being selected from the group of Na, Li, and K, for broadening the
amplification wavelength band of the optical amplifier fiber.

9. The amplifier fiber according to claim 8, wherein the glass composition
further comprises up to 45 mole % PbO.

10. The amplifier fiber according to claim 8, wherein the glass component
amounts are such that the amplification wavelength band is at least 250
nm.

15. An optical amplifier (20) including an optical amplifier fiber (28)
comprising a core surrounded by at least one cladding, wherein the core
is at least partly formed of a germanate glass composition doped with
Tm3+ and Ho3+ and further comprising at least 35 mole %
GeO2 and oxides of Ba, Zn and R, R being selected from the group of
Na, Li, and K, for broadening the amplification wavelength band of the
optical amplifier fiber.

17. The optical amplifier according to claim 15, forming a complementary
amplifier for side-band amplification.

18. A laser device (30) including an optical amplifier fiber (38), wherein
the optical amplifier fiber is at least partly formed of a germanate
glass composition doped with Tm3+ and Ho3+ and further
comprising at least 35 mole % GeO2 and oxides of Ba, Zn and R, R
being selected from the group of Na, Li, and K, for broadening the
amplification wavelength band of the optical amplifier fiber.

Description:

TECHNICAL FIELD

[0001]The present invention relates to optical fibers and in particular to
a glass composition suitable for use in a fiber amplifier for
amplification of optical signals.

BACKGROUND

[0002]Telecommunication networks of today generally employ optical fibers
for signal transmission. Optical signals are transported long distances
on optical carriers and features like long legs and power splitting
necessitate amplification of weakened signals. Optical amplifiers
typically comprise a comparatively short amplifier fiber doped with a
rare-earth ion or another substance that is capable of fluorescing. Light
from a pump source causes electrons of the rare-earth ions to jump to a
temporary excited stage, and light of the input signal stimulates
spontaneous emission from the excited level. The light of this emission
presents the same characteristics (wavelength, polarization and direction
of propagation) as the input signal and the emission results in that the
gain of the input signal is increased.

[0003]The demand for increasing bandwidth, primarily caused by the
tremendous growth of the Internet, is driving the rapid deployment of
optical amplifiers. For the conventional (C) band, it is well known to
use erbium doped fiber amplifiers (EDFA), which has been thoroughly
researched. However, the increasing demand for bandwidth in wavelength
division multiplexing (WDM) optical communication systems has lead
towards extending the transmission bands outside the C-band. Below the
C-band, there is the so-called S-band (1460-1520 nm) for which the more
recent thulium doped fiber amplifiers (TDFA) are suitable. There are also
new EDFAs, operating at a record 25 dBm output power, which have a gain
flatness of less than 0.8 dB over the L-band (1570-1610 nm).

[0004]Both EDFA and TDFA use fibers doped with rare earth ions that
typically show a bandwidth of approximately 90 nm. In [1], for example,
an Er3+/Tm3+ codoped silica fiber with a bandwidth from 1460 to
1550 nm is described. The maximum values of optical amplifier bandwidth
presented, all kinds of glass compositions considered, lie in the range
of 110-130 nm [2, 3]. In order to achieve true broadband amplification,
this bandwidth is not sufficient. It would be very desirable to find a
way of improving the bandwidth of optical amplifiers based on rare earth
doped fibers.

[0005]Moreover, a problem associated with previous fiber types, such as
fibers based on fluoride, tellurite and chalcogenide glasses, are the
inferior mechanical properties thereof. Such fibers are often
incompatible with the conventional silica fibers used in telecom.

[0006]Accordingly, the optical amplifier fibers of conventional
telecommunication systems are far from satisfactory and there is a
considerable need for an improved glass composition allowing broadband
amplification of optical signals.

SUMMARY

[0007]A general object of the present invention is to provide improved
optical amplifier fibers. A specific object is to improve the bandwidth
of optical amplifier fibers doped with rare-earth ions. Another object is
to achieve a broadband optical amplifier fiber that easily can be
implemented together with conventional telecommunication systems.

[0008]These objects are achieved in accordance with the attached claims.

[0009]Briefly, a new germanate (GeO2) glass composition suitable for
broadband optical amplifier fibers is provided. The glass is doped with
thulium (Tm3+) and co-doped with holmium (Ho3+). The glass
includes at least 35 mole % GeO2, as well as the metal oxides BaO,
ZnO, and R2O (R=Na, Li, K). Preferably, up to 45 mole % PbO is also
included. A preferred embodiment uses the glass
50GeO2-25PbO-10BaO-10ZnO-5K2O. The glass composition of the
invention results in a considerably broadened bandwidth as compared to
previous glasses. A bandwidth of about 310 nm is achieved, enabling for a
new kind of broadband optical amplifiers and operation in the S++,
S+, S, C, L and L+ bands at the same time. Another advantage is
that the germanate glass of the invention has similar properties as
silica glasses, which makes the fibers highly compatible with the silica
optical fibers used for signal transmission in conventional communication
systems.

[0010]According to other aspects of the present invention, an optical
amplifier fiber, an optical amplifier, and a laser device are provided.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011]The invention, together with further objects and advantages thereof,
is best understood by reference to the following description and the
accompanying drawings, in which:

[0012]FIG. 1 is a schematic cross-sectional view of an exemplary
embodiment of an optical amplifier fiber in accordance with the present
invention;

[0013]FIG. 2 contains an emission spectrum for an exemplary rare-earth
doped germanate glass composition in accordance with the present
invention;

[0014]FIG. 3 is a schematic block diagram of an exemplary embodiment of an
optical amplifier in accordance with the present invention; and

[0015]FIG. 4 is a schematic block diagram of an exemplary embodiment of a
laser device in accordance with the present invention.

DETAILED DESCRIPTION

[0016]FIG. 1 illustrates the basic structure of a typical fiber-optic
cable. An optical fiber 10 comprising a core 12 and a cladding 14 is
shown. The core 12 is a transparent glass material through which the
light travels. It is surrounded by another glass sheet, the cladding 14,
which generally has a refractive index lower than that of the core. The
cladding acts like a mirror, reflecting light back into the core, and the
light is thus transmitted through the optical fiber 10 by means of
internal reflection. The outer side of the optical fiber 10 is covered
with a protective coating 16 of an insulating material.

[0017]If the optical fiber is used for amplifying purposes, the core glass
is generally doped with a substance that is capable of fluorescing, such
as a rare earth ion. Light from a pump source causes electrons of the
rare-earth ions to jump to a temporary excited stage, and the light of
the input signal stimulates spontaneous emission of the excited level.
The light from this emission has the same characteristics (wavelength,
polarization and direction of propagation) as the input signal and the
emission results in that the gain of the input signal is increased.

[0018]As mentioned in the background section, a problem with glasses for
optical amplifier fibers is that they are associated with too narrow
bandwidth to allow efficient and wide broadband emission. A glass
allowing amplification in the S++, S+, S, C, L and L+
bands at the same time would be very desirable. However, the best optical
amplifiers that have been presented in the prior-art have a bandwidth
around 110-130 nm [2, 3]. To our knowledge, 130 nm is the maximum value
presented, irrespective of glass composition.

[0019]The present invention proposes a new glass composition that has
shown to provide outstanding amplification properties and a considerably
increased bandwidth as compared to previous glasses. This is achieved by
means of a germanate glass comprising an advantageous combination of
metal oxides. More specifically, the glass composition according to the
invention includes at least 35 mole % GeO2, 0-45 mole % PbO, as well
as appropriate amounts of the metal oxides BaO, ZnO, and R2O (R=Na,
Li, K). The glass is doped with the rare earth ions Tm3+ and
Ho3+.

[0020]The new type of broadband emission achieved with the present
invention is illustrated by FIG. 2, which shows a spectrum for the
proposed germanate glass codoped with Tm3+/Ho3+. Pumping at 488
nm, 300K, a luminescence broadband is observed from 390 nm to 700 nm,
i.e. over 310 nm at half the height. This means that the germanate glass
of the invention can be used for amplification in the S++, S+,
S, C, L and L+ bands at the same time.

[0021]Recalling that the maximum bandwidth in the prior art is around 130
nm, FIG. 2 shows that the proposed glass composition presents a bandwidth
of 310 nm for the same spectral range. Thus, the invention results in a
more than doubled bandwidth, or an increase of about 140%! The broadband
amplified spontaneous emission obtained with the invention enables a new
type of broadband amplifier capable of operating in bandwidth range of up
to 310 nm.

[0022]Preferred glasses in accordance with the invention thus provide an
amplification wavelength band of 310 nm and in any case a bandwidth of
250 nm or more can easily be obtained by means of the invention.

[0023]The large bandwidth of the proposed germanate glass is due to the
favorable spectroscopic properties of the host composition. The host
composition creates a non-homogeneous network where the dopant ions
(Tm3+ and Ho3+) are enclosed. Each ion responds differently to
the surrounding environment and this may cause broadening of the band
emission. The total bandwidth is the overall summation of emissions for
all dopant ions present. The glass of the invention provides an excellent
environment for the Tm3+ and Ho3+ ions and results in an
improved shift amplitude i.e. an improved bandwidth compared to
previously known glasses.

[0024]A major advantage of the present invention is that the proposed new
glass fibers are associated with the same characteristics as silica
fibers. By using fibers based on a germanate glass, the mechanical
properties become much better that for fluoride, tellurite and other
heavy metal oxide glasses in the sense that they resemble those of
conventional silica fibers. Hence, the fibers according to the invention
are highly compatible with existing silica fibers.

[0025]Other advantages of the new glass are that it presents a high
thermal stability against devitrification as well as a high viscosity.
These parameters are crucial during the drawing process to manufacture
optical fibers.

[0026]Table 1 contains approximate mole % values for a preferred germanate
glass in accordance with the present invention. The glass can very well
also present other mole % values, such as the glasses
(75-X)GeO2--(X)PbO-10BaO-10ZnO-5K2O-0.2Tm3+-0.2Ho3+,
with X between 0 and 40. Table 2 contains preferred mole % ranges for the
components of germanate glasses in accordance with the present invention.

[0027]As illustrated by the above tables, the proposed glass preferably
contains K2O, but one or more other alkalimetal oxides may also be
used, more specifically R2O, where R=Na, Li, and/or K.

[0028]It should be mentioned that germanate glasses have been used as
hosts for Tm3+ in the prior art. An example is the glass fiber of
[3] comprising GeO2 and Ga2O3 for operation in the
1460-1530 nm wavelength band.

[0029]An optical amplifier fiber according to the invention may with
advantage present the basic structure, which was described above with
reference to FIG. 1. In other words, FIG. 1 is a schematic
cross-sectional view of an exemplary embodiment of an optical fiber in
accordance with the present invention. The optical fiber 10 comprises the
core 12, the cladding 14 and preferably also the protective coating 16.
The core glass comprises the above-described germanate glass host,
preferably (35-75)GeO2-(0-45)PbO-(5-20)BaO-(5-20)ZnO-(2-10)R2O,
and is doped with a first lanthanide oxide (Tm2O3) and codoped
with a second lanthanide oxide (Ho2O3).

[0030]It is important that the glass compositions of the core and cladding
are about the same to avoid a significant expansion coefficient mismatch.
The same germanate glass composition can for example be used as a base
for both the core 12 and the cladding 14 of the optical fiber 10. The
core glass can then be doped and modified to contain a higher amount of a
substance used for refractive index control.

[0031]It should be noted that the optical fiber structure of FIG. 1 is
rather simplified. Other optical fibers in accordance with the invention
may present more complex structures with non-symmetrical components,
graded-index cores, more than one cladding, etc.

[0032]FIG. 3 is a schematic block diagram of an exemplary embodiment of an
optical amplifier in accordance with the present invention. The
illustrated optical amplifier 20 comprises signal processing means 22,
couplers 24, pump light sources 26 and an optical amplifier fiber 28. A
weak optical signal that needs to be amplified is input to the amplifier
20. The input signal first passes the optional signal processing means
22a, which modifies the signal in an appropriate way. The amplifier fiber
28 is pumped at both ends with pump lasers or similar pump light sources
26a and 26b (λexc=488 nm or 800 nm). The couplers 24a and 24b
combine the excitation light provided by the pump light sources with the
signal light. In the amplifier fiber 28, the excitation light then causes
the rare-earth ions (Tm3+, Ho3+) to attain a temporary excited
state. As the electrons decay, light with the same characteristics as the
input signal is emitted and the gain of the optical signal is thus
increased. Finally, the amplified signal is further modified in the
optional signal processing means 22b. A comparatively strong optical
signal is output from the amplifier.

[0033]The optical amplifier fiber 28 of the optical amplifier 20 comprises
a core of a germanate glass doped with rare earth ions. The core (and
possible also the cladding) includes the germanate glass host of the
invention, e.g.
(35-75)GeO2-(0-45)PbO-(5-20)BaO-(5-20)ZnO-(2-10)R2O. The core
is doped with Tm3+ and Ho3+ (0.01-2.5%).

[0034]The signal processing means 22 preferably comprises isolators, the
purposes of which are to prevent unwanted reflections and suppress the
oscillations of the amplifier. The signal processing means 22 may also
include further devices for modulation, filtering, polarization,
absorption, attenuation, etc.

[0035]The optical amplifier according to FIG. 3 may of course be subject
to various modifications obvious to the skilled man. It would for
instance be possible to use a single pump light source (and a single
coupler), even though two pump light sources generally result in better
amplifier efficiency. The number and position of the optional signal
processing means units may vary and filters and the like can be either
internal or external. There may further be more than one amplifier fiber
in the optical amplifier. Besides the amplifier fiber(s), there are
generally several undoped "ordinary" optical fibers in the optical
amplifier, providing connections between components thereof.

[0036]Additionally, the optical amplifier of the invention may also be
employed as a complementary amplifier for side-band amplification in EDFA
networks. In this case, the gain of the spectral range corresponding to
the side band will be 50% improved. This represents a new and very
advantageous application for the optical amplifier glass fiber.

[0037]The above-described optical amplifier glass may also be used in
other optical devices, such as solid state lasers (where the active
medium is a glass rod), active waveguides, infrared sensors, etc. FIG. 4
is a schematic block diagram of an exemplary embodiment of a fiber laser
device in accordance with the present invention. Each laser device
component is provided with the same reference number as the corresponding
amplifier component (FIG. 3) plus 10. The main difference between the
laser device and the optical amplifier is that the laser device does not
receive a signal light input but has feedback means for signal
generation. The laser device 30 of FIG. 4 accordingly comprises two
reflectors 35, placed at opposite ends of the optical amplifier fiber 38.
Excitation light from the pump light sources 36 give rise to photon
emission in the amplifier fiber 38 in the same way as for the optical
amplifier. The first reflector 35a is preferably a high reflector mirror,
ideally reflecting all light, whereas the second reflector 35b is a
partially transparent mirror. The relatively small fraction of light
passing through the second reflector is the laser beam output of the
laser device 30. Optional signal processing means 32 of the laser device
may include internal or external devices for modulation, filtering,
polarization, q-switching, absorption and the like.

[0038]In another embodiment (not shown) of the laser device according to
the invention, feedback is instead achieved by a ring-shaped fiber
structure, where a part of the output signal basically is led back to the
coupler 34a.

[0039]Although the invention has been described with reference to specific
illustrated embodiments, it should be emphasized that it also covers
equivalents to the disclosed features, as well as modifications and
variants obvious to a man skilled in the art. Thus, the scope of the
invention is only limited by the enclosed claims.